Engine induction systems are sometimes tuned to facilitate the flow of the air through the induction system into the cylinders of the engine. The improvement in the charging of the cylinders is produced by constructing the components of the induction system so that resonances and momentum increases are created in the air flow at one or more engine speeds. When resonances and momentum increases are created in the air flow in the induction system, the pressure at the cylinder intake ports increases enabling an increased charge of air to enter the cylinders. The improvement in the charging characteristics of the induction system depends on various factors including the size and number of plenums, the number and length of the runners which feed each cylinder, and the circuitry interconnecting the plenums, runners and cylinders.
Tuned induction systems are sometimes constructed with a single plenum which feeds the cylinders of an engine through a single runner extending to each cylinder. A problem associated with such systems is that the engine speeds at which cylinder charging is improved from such tuning are limited. In addition, the large plenum volume, necessary to accommodate the flow through the runners at high engine speeds, reduces the throttle responsiveness and tuning effectiveness at low engine speeds.
Induction systems are also constructed with a single plenum which feeds each cylinder through two or more individual runners. One runner, constituting the primary runner, is tuned for low engine speeds. The other runners, constituting the secondary runners, are tuned for high engine speeds. Valves are located in the secondary runners to obstruct flow through them during low speeds so that the induction system can be tuned at a low engine speed due to the tuning of the primary runners. At higher engine speeds, the valves are opened so that the air flows through the secondary runners in addition to the primary runners. The induction system can thereby be tuned at a higher engine speed due to the tuning of the secondary runners.
Several problems are associated with this type of induction system. First, since there is only a single plenum, it must be sized to accommodate flow through all of the runners at high engine speeds. As discussed above, large plenum volumes reduce the throttle responsiveness and tuning effectiveness at low engine speeds. Second, the air can flow into the secondary runners at low engine speeds, even though the valves in the runners are closed, since only a single valve is located in each runner. If the valves are located in the outlets of the secondary runners, then the secondary runners communicate with the plenum through their inlets and thereby act as extensions of the plenum to increase the effective plenum volume of the induction system. If the valves are located in the inlets of the secondary runners, then the air in the induction system can enter the secondary runners through their outlets as, for example, when the cylinder intake valve is closed, causing resonances in the secondary runners which interfere with the tuning of the primary runners.
Induction systems are sometimes constructed with separate plenums which feed each cylinder through separate runners. The plenums are interconnected so that a single upstream throttle can control air flow into both plenums. One runner, constituting the primary runner, is tuned for low engine speeds and the other runner, constituting the secondary runner, is tuned for high engine speeds. An outlet valve is connected between each pair of primary and secondary runner outlets so that, at low engine speeds, each outlet valve obstructs a secondary runner outlet and, at high engine speeds, each outlet valve obstructs a primary runner outlet.
Several problems are associated with this type of induction system. First, at low engine speeds when the secondary runner outlets are obstructed, the secondary runners and plenum continue to communicate with the primary plenum since the primary and secondary plenums are connected and the secondary runner inlets are unobstructed. This results in the induction system having a larger effective plenum volume which reduces the throttle responsiveness and tuning effectiveness at low engine speeds, as discussed above. Second, since the primary runner is closed at high engine speeds, the secondary plenum and runners must be sufficiently large to accommodate the increased flow associated with high engine speeds and the flow diverted from the primary runners. Closing the primary runners at high engine speeds also prevents the tuning of the primary runners from supplementing the tuning of the secondary runners.